Image intensifier type camera tube with potential field correcting means



Sept. 20, 1966 ROTOW 3,274,416

IMAGE INTENSIFIER TYPE CAMERA TUBE WITH POTENTIAL FIELD CORRECTING MEANS Filed Sept. 27, 1961 2 Sheets-Sheet 1 fra *20 0 70*2001 INVENT R.

BY .25. 412W Sept. 20, 1966 A A. ROTOW 3,274,416

IMAGE INTENSIFIER TYPE CAMERA TUBE WITH POTENTIAL FIELD CORRECTING MEANS Filed Sept. 27, 1961 2 Sheets-Sheet 2 INVENTOR. flaw/win. if Poi-aw United States Patent 3,274,416 IMAGE INTENSIFIER TYPE CAMERA TUBE WITH POTENTIAL FIELD CGRRECTING MEANS.

Alexander A. Rotow, Lancaster, Pa., assignor, by mesne assignments, to the United States of America as represented by the Secretary of the Army Filed Sept. 27, 1961, Ser. No. 141,135 5 Claims. (Cl. 313-65) This invention relates to imaging tubes. In particular, this invention relates to image intensifying tubes of the type wherein a photoelectron image is intensified within a tube, one or more times, prior to the time when the image is visably reproduced or prior to the time when the image is formed into electrical output signals of a television pickup tube.

In the prior art, there are certain tubes known as image intensifiers. Generally these tubes comprise a first photoemissive surface which produces a first photoelectron image of a scene to be reproduced. The first photoelectron image is focused and accelerated to land on a first phosphor screen to produce light. This area of the tube is usually referred to as an image section. The light produced by the phosphor screen is coupled to a second photoemissive surface to produce a second photoelectron image. The second photelectron image is an intensified image of the first photoelectron image and corresponds to the light from the scene. The second, or intensified, photoelectron image may be directed onto a phosphor screen to produce visible output signals; or electrons from the second, intensified photoelectron image may be directed into an electron multiplier structure to produce electrical output signals, as in a pickup tube; or, the second, intensified photoelectron image may be directed to another electron intensifier or multiplying structure to produce a third and still further intensified photoelectron image.

In the tubes of the type briefly described above, the photoelectron image from the first photocathode is normally magnetically focused and electrostatically accelerated. Photoelectrons emitted from a point on the first photocathode follow a helical path and should be focused onto a corresponding point on the first phosphor screen.

Unfortunately, for a uniform electrical field gradient and when using an electron accelerating voltage of approximately 20,000 volts (which is approximately the potential difference that is used for a bright picture to be produced on a conventional phosphor screen, and when using a magnetic field of approximately 75 gausses) the distance between the photocathode and the phosphor screen, for one complete loop of focus, is about 7 /2 to 8 inches. This length, together with a standard Image Orthicon type pickup tube, makes necessary, when designing an image intensifying camera tube, a tube of about 23 inches in length. Such a tube requires a focusing coil of about 19 inches in length approximately 30,000 turns of wire with about 0.075 amperes passing through it for proper focusing. The dimensions, weight, and power requirements of such an installation are larger than desirable. One way of reducing the prohibitive length of this type of tube is described in copending application Serial Number 841,508, filed September 22, 1959 now US. Patent Number 3,047,757 issued August 31, 1962 by A. A. Rotow and assigned to the assignee of the present invention. The solution to the length problem described in the above identified application includes the use of a fine mesh screen electrode positioned between the first photocathode and the first phosphor screen. Such a mesh screen lowers the sensitivity of the tube by about 35 percent since the transparency of such a mesh screen, to the photoelectron image, is about 65 percent.

Another solution proposed to the prohibitive length problem in this type of tube is to provide a conventional electrostatic lens system, in conjunction with the conventional magnetic focusing means, between the photocathode and the phosphor screen. This combination should focus the photo electrons onto the phosphor screen in a relatively short distance. However, when this proposed solution was tried, the picture was badly distorted in areas removed from the tube axis due to the very strong curvature of the electrostatic equipotential surfaces provided by the lens.

It is therefore an object of this invention to provide an improved imaging device.

It is another object of this invention to provide a novel image intensifying tube characterized in its relatively short length and its freedom from image distortion.

These and other objects are accomplished in accordance with this invention by providing an image intensifying device which includes a plurality of field correcting electrodes in the image section of the tube. Each of the field correcting electrodes is located an appropriate distance from the photocathode and each has as large a diameter as the tube geometry permits. The field correcting electrodes influence the electrostatic field at the edges of the picture without noticeably changing the field close to the tube axis. Also, in accordance with another feature of this invention, the mesh screen structure, described in the above identified application, can be combined with the field correcting electrodes to provide a still greater improvement in picture resolution, definition, and image intensification While using relatively short tube structures.

The invention will be more clearly understood by reference to the accompanying drawings wherein:

FIG. 1 is a partially sectional view of an image intensi fier Image Orthicon in accordance with this invention;

FIG. 2 is a partially sectional view of another embodiment of an image intensifier Image Orthicon in accordance with this invention; and,

FIG. 3 is a partially sectional view of still another embodiment of this invention.

Referring now to the drawings in more detail, FIG. 1 illustrates an image intensifier Image Orthicon tube 10 in which this invention is particularly applicable. The invention is also useful in other image intensifying types of tubes, such as image intensifying image tubes or image intensifying tubes having more than one stage of image intensification.

The image intensifier tube 10 comprises an evacuated envelope 12 having an electron gun (not shown) positioned in the stem end (the left hand end as viewed in FIG. 1) thereof. The electron gun may be any conventional structure and is not shown for simplicity of illustration. Positioned in the other end of the envelope 12 is a first or input photocathode 14. Positioned in the path of the photoelectron image from the photocathode 14 is an image intensifying sandwich 16. The image intensifying sandwich 16 may comprise any conventional structure such as a thin transparent support member 18, eg glass, having a phosphor screen 20 positioned on the side of support member 18 toward or facing the photocathode 14. The phosphor screen may be of any suitable phosphor material, e.g. Zinc sulfide. The phosphor screen 20 is aluminized by applying an aluminous coating 21 in known manner on the side of the screen 20 facing the photocathode 14. A second photocathode 22 is positioned on the opposite side of the support member 18.

When electrons from the first photocathode 14 strike the phosphor screen 20, a visible image is produced. The light from the phosphor screen 20 excites the second photocathode 22. Due to this excitation of the second photocathode 22, a second photoelectron image is produced. The second photoelectron image is accelerated and lands on one side of a thin storage target electrode 24. Thus an intensified charge .pattern is developed on the target electrode 24 which corresponds to the original light from the scene. The electron beam from the electron gun scans the target electrode 24 to remove the charge pattern therefrom. In so doing, a return electron beam is provided which is directed into a conventional electron multiplier (not shown) to provide electrical output signals from the image intensifier Image Orthicon 10. The operation of an Image Orthicon tube is well known and is therefore not described in detail.

The target electrode 24 may, for example, be a thin membrane of glass or magnesium oxidev The photocathodes 14 and 22 may be of suitable, known materials such as the well known S-lO photosurface, described in US. Patent No. 2,682,479 to Johnson, or the multialkali photosurface described in US. Patent No. 2,770,561 to Sommers. The phosphor screen 20 may be any suitable known material such as zinc sulfide or may be of the evaporated phosphor, e.g. rubidium iodide.

It should be understood that the tube 19 is shown merely as an example of a tube structure in which this invention is particularly useful. This invention is also applicable to tubes wherein another intensifier sandwich, similar to sandwich 16, is provided between the target 24 and the intensifier sandwich 16. Also, the charge storage target 24 may be replaced by a phosphor output screen to provide visual output information in an image intensifying image tube.

The problem of the prior art structures is that the magnetic field provided by a focus coil 26, and illustrated by the arrows in FIG. 1, must be extremely strong in order to complete at least one loop of focus in the space existing between the photocathode 14 and the aluminized phosphor screen 20. The trajectory of the photoelectrons between the photocathode 14 and the aluminized phosphor screen 20 is in the form of a helix, such as the helix 23 as shown, that is tangent to the magnetic lines of force. In order to obtain sharp focus, an integral number of turns of these helices must be exactly completed by the time the electrons reach the phosphor screen 20. In other words, at least one complete loop of focus must be obtained. To obtain this result the magnetic field intensity and the applied voltages must be in a definite relationship.

Generally, the electric and magnetic fields are adjusted for the best possible focus in the middle of the picture. These adjustments normally cause a substantial loss of resolution in the corners of the pictures. In order to overcome this loss of resolution, there is provided two potential-correcting electrodes or rings 30 and 32. It is important that the potential-correcting rings 30 and 32 should not affect, by a considerable amount, the potential distribution that exists along the axis of the tube 10. This field distribution is achieved by making the length in the axial direction of the ring of each of the potential correcting rings 30 and 32 small as compared to the diameter of each ring. That is, the axial length of the potentialcorrecting rings should be not more than percent of the internal diameter of the rings and can be susbtantially smaller.

During operation of the tube '10, the potential-correcting rings 30 and 32 are held at a potential, such as that shown in FIG. 1, that is higher than the potential of the first photocathode 14 or may be that of the axially symmetric electrode or wall coating 33. The effect caused by the potential correcting rings 30 and 32, upon the electric field, is that the electrostatic field is made more intense adjacent the photocathode 14 near the envelope wall, i.e. near the outer periphery of the photocathode 14. Thus, the equipotential lines 31 are pulled outwardly, particularly in the regions closely adjacent the envelope wall, which results in the equipotential field lines 31 being substantially fiat and substantially parallel to the photocathode 14.

The result is that photoelectrons from the center of the photocathode 14 (path 23) and the photoelectrons from the corners of the photocathode 14 (path 19), cover approximately the same length of trajectory between two equipotential surfaces, i.e. between the photocathode 14 and the plane of the sandwich 16.

Referring now to FIG. 2, there is shown an embodiment of this invention in which mesh screen 34 is utilized in accordance with the teachings of the above identified Rotow application, in combination with field correcting rings. In this embodiment, the mesh screen 34 is positioned between the first photocathode 14 and the amplifying sandwich 16. A plurality of field correcting rings 38, 39, 40 and 41 are provided on the photocathode 14 side of the mesh screen 34. Also, a plurality of field correcting rings, 42, 43, 44, and 45 are provided on the sandwich 16 side of the mesh screen 34. During operation, the same potential may be applied to the alternate field correcting electrodes 43 and 45 on one side of the mesh screen 34 as is applied to the alternate field correcting electrodes 39 and 41 on the opposite side of the mesh screen 34. In this instance a smaller potential is applied to the field correcting electrodes 42 and 44 while a larger potential, that of photocathode 14, is applied to the field correcting electrodes 39 and 41. By means of this geometry and these potentials, the electric field near the mesh screen 34 is substantially lower than when no potential correcting rings are used. Thus, the likelihood of cold electron emission from the mesh screen 34 is decreased. Also, the mesh screen 34 may now be located in an area in which the mesh screen 34 is more out of focus which allows the mesh screen 34 to be made much coarser and hence with a higher mesh transparency (up to percent). Still further, the arrangement of field correcting rings provides an electrostatic field that is substantially the same adjacent to the edges of the tube adjacent the photocathode 14 as it is adjacent to the tube axis.

In the embodiment shown in FIG. 3, potential correcting rings 52 and 54 are positioned within a highly resistive spiral coating 50. The spiral coating 50 may be made of a material such as graphite and may have a resistivity of approximately 10 ohm centimeters. By means of the spiral coating 50 a continuous potential difference is provided throughout the length of the image section, i.e. between the photocathode 14 and the amplifying sandwich 16. The potential correcting rings 52 and 54 flatten the equipotential lines of force when potentials such as those shown in FIG. 3 are applied thereto.

In each of the embodiments of this invention there is provided a means for decreasing the length of an image intensifier section while providing a complete loop of focus for the photoelectrons from all areas of the photocathode. Thus, corner resolution is improved and image distortion and rotation are substantially decreased.

What is claimed is:

1. An image intensifying camera tube comprising an evacuated envelope, a photoemissive cathode in one end of said envelope, an amplifying sandwich in said envelope and spaced from said photoemissive cathode, means for accelerating photoelectrons from said photoemissive cathode to said amplifying sandwich, focusing means for focusing said photoelectrons onto said amplifying sandwich, said focusing means including at least two fieldcorrecting electrodes positioned between said photoemissive cathode and said sandwich, said field-correcting electrodes being positioned around the path of said photoelectrons in their transit from said photoemissive cathode to said amplifying sandwich, a conductive wall coating spaced around said field-correcting electrodes and extending from said photoemissive cathode to a plane parallel and adjacent to said amplifying sandwich, said conductive wall coating being adapted to have a first voltage applied thereto, the field-correcting electrode adjacent to said photoemissive cathode adapted to have a second voltage applied thereto, said second voltage being substantially more positive than said first voltage, the field-correcting electrode adjacent to said amplifying sandwich being adapted to have a third voltage applied thereto, and said third voltage being substantially more positive than said second voltage.

2. An image intensifying device comprising an evacuated envelope having a photocathode therein, an amplifying sandwich spaced from said photocathode and in said envelope, means for focusing electrons from said photocathode onto said amplifying sandwich, said means comprising a plurality of field-correcting electrodes and a mesh screen, each of said field-correcting electrodes being an annular ring-like member having an axial length not greater than of the diameter thereof, and at least one of said plurality of field correcting electrodes being positioned on one side of said mesh screen with the balance of said plurality field-correcting electrodes being positioned on the other side of said mesh screen.

3. A pickup tube comprising an evacuated envelope having a photoemissive cathode in one end thereof, means for focusing photoelectrons from said photoemissive cathode onto a plane spaced from said photoemissive cathode, said focusing means including a spiraled continuous coating of resistive material extending from adjacent said photoemissive cathode to adjacent said plane, said focusing means further comprising at least two field correcting ring electrodes each having an axial length of not more than one-tenth the diameter thereof.

4. An image section of an image tube comprising a planar photocathode, means for focusing photoelectrons from said photocathode in a focal plane removed from and parallel to the plane of said photocathode, said means comprising a mesh screen, said means further comprising at least two potential field-correcting rings positioned on one side of said mesh screen, and at least two potential field-correcting rings positioned on the opposite side of said mesh screen, all of said potential field-correcting rings surrounding the space between said photocathode and said focal plane through which space said photoelectrons travel in their transit to said plane each field correcting ring having an axial dimension no longer than onetenth of the diameter thereof.

5. An image intensifying section of an image sensitive device comprising a planar shaped photocathode, a focal plane for photoelectrons from said photocathode, a tubular electrode having one end adjacent to said photocathode and the other end adjacent to said focal plane, said focal plane being substantially parallel to and spaced from said photocathode, at least two potential field-correcting electrodes spaced inwardly from said tubular electrode and around the path of photoelectrons in their transit from said photocathode to said focal plane.

References Cited by the Examiner UNITED STATES PATENTS 2,258,294 10/1941 Lubszynski et a1. 313-66 2,550,316 4/1951 Wilder 313-66 FOREIGN PATENTS 615,563 4/1945 Great Britain.

JAMES W. LAWRENCE, Primary Examiner.

ARTHUR GAUSS, KATHLEEN CLAFFY, Examiners.

L. LAFRANCHI, S. CHATMON, JR.,

Assistant Examiners. 

5. AN IMAGE INTENSIFYING SECTION OF AN IMAGE SENSITIVE DEVICE COMPRISING A PLANAR SHAPED PHOTOCATHODE, A FOCAL PLANE FOR PHOTOELECTRONS FROM SAID PHOTOCATHODE, A TUBULAR ELECTRODE HAVING ONE END ADJACENT TO SAID PHOTOCATHODE AND THE OTHER END ADJACENT TO SAID FOCAL PLANE, SAID FOCAL PLANE BEING SUBSTANTIALLY PARALLEL TO AND SPACED FROM SAID PHOTOCATHODE, AT LEAST TWO POTENTIAL FIELD-CORRECTING ELECTRODES SPACED INWARDLY FROM SAID TUBULAR ELEC- 